A system has been devised which allows road toll fees to be collected automatically, the system comprising an interrogator device buried in the carriageway and transponder tags fitted to vehicles.
When a suitably equipped vehicle approaches a toll point its presence is detected by an inductive loop detector (similar to that used at traffic lights) and the interrogator is energised. The interrogator transmits to the vehicle a signal at 915 MHz carrying a code which is recognised by the vehicle tag as a command to turn on its transmitter. The tag transmitter then sends a coded signal identifying the vehicle registration number to the interrogator in the carriageway. It is therefore possible to collect toll fees for tag-equipped vehicles without the need for manned toll booths and without the need for the vehicles to slow down so that money could be transferred.
It will be understood that for the purposes of this specification, "transponder tag" is intended to mean a unitary device which includes elements necessary for this operation in receiving radiated signals and transmitting signals in response thereto.